Agile spread waveform generator

Abstract

An agile spread spectrum waveform generator comprises a photonic oscillator and an optical heterodyne synthesizer. The photonic oscillator comprises a multi-tone optical comb generator for generating a series of RF comb lines on an optical carrier. The optical heterodyne synthesizer includes first and second phase-locked lasers; the first laser feeding the multi-tone optical comb generator and the second laser comprising a rapidly wavelength-tunable single tone laser whose output light provides a frequency translation reference. A photodetector is provided for heterodyning the frequency translation reference with the optical output of the photonic oscillator to generate an agile spread spectrum waveform.

1. An agile spread spectrum waveform generator comprising:
 (a) a photonic oscillator comprising a multi-tone optical comb generator for generating a series of RF comb lines on an optical carrier; 
 (b) an optical heterodyne synthesizer, the optical heterodyne synthesizer including first and second phase-locked lasers, the first laser feeding the optical carrier to the multi-tone optical comb generator and the second laser comprising a wavelength-tunable single tone laser whose output light provides a frequency translation reference; and 
 (c) a photodetector for heterodyning the frequency translation reference with the series of RF comb lines on the optical carrier generated by the photonic oscillator to generate an agile spread spectrum RF waveform; 
 wherein the photonic oscillator further comprises multiple loops including: 
 (i) a first optical delay line in a first loop for spacing a comb generated by the multi-tone optical comb generator; 
 (ii) a second optical delay in a second loop line for noise reduction, the second delay line being longer than the first optical delay line; 
 (iii) at least one photodetector connected to the first and second delay lines; and 
 (iv) an optical intensity modulator in a loop portion common to the first and second loops for driving the first and second optical delay lines. 
 
   
   
     2. The agile spread spectrum waveform generator of  claim 1  wherein the loop common portion further includes an amplifier and a band pass filter. 
   
   
     3. The agile spread spectrum waveform generator of  claim 2  wherein the amplifier is an electronic amplifier. 
   
   
     4. The agile spread spectrum waveform generator of  claim 1  wherein the loop common portion further includes a band pass filter and wherein at least one of the first and second loops includes an optical amplifier therein. 
   
   
     5. The agile spread spectrum waveform generator of  claim 1  further including means for compensating for environmental changes affecting a length of at least one of the first and second optical delay lines. 
   
   
     6. The agile spread spectrum waveform generator of  claim 5  wherein the means for compensating for environmental changes affecting the length of at least one of the first and second optical delay lines comprises an apparatus for adjusting the length of at least one of the first and second optical delay lines and a feedback circuit including a tone selection filter to the loop common portion and a mixer for mixing the output of the tone selection filter with a reference signal, an output of the mixer being operatively coupled to the length adjusting apparatus. 
   
   
     7. The agile spread spectrum waveform generator of  claim 6  wherein the tone selection filter is coupled to the optical intensity modulator. 
   
   
     8. The agile spread spectrum waveform generator of  claim 7  wherein the optical intensity modulator is an electro-absorption modulator having an electrical output coupled to the tone selection filter. 
   
   
     9. The agile spread spectrum waveform generator of  claim 7  wherein the tone selection filter receives signals from the optical intensity modulator. 
   
   
     10. The agile spread spectrum waveform generator of  claim 6  wherein the length adjusting apparatus adjusts the length of both of the first and second optical delay lines. 
   
   
     11. The agile spread spectrum waveform generator of  claim 5  wherein the means for compensating for environmental changes affecting the length of at least one of the first and second optical delay lines comprises a phase shifter disposed in the loop common portion and a feedback circuit including a tone selection filter coupled to the loop common portion and a mixer for mixing the output of the tone selection filter with a reference signal, an output of the mixer being operatively coupled to the phase shifter. 
   
   
     12. The agile spread spectrum waveform generator of  claim 11  wherein the tone selection filter is coupled to the optical intensity modulator. 
   
   
     13. The agile spread spectrum waveform generator of  claim 12  wherein the optical intensity modulator is an electro-absorption modulator having an electrical output coupled to the tone selection filter. 
   
   
     14. The agile spread spectrum waveform generator of  claim 11  wherein the tone selection filter receives signals from the optical intensity modulator. 
   
   
     15. The agile spread spectrum waveform generator of  claim 1  further including a injection seeding circuit for seeding the photonic oscillator. 
   
   
     16. The agile spread spectrum waveform generator of  claim 1  wherein the second optical delay line is more than 40 times longer than is the first optical delay line. 
   
   
     17. The agile spread spectrum waveform generator of  claim 1  further including an optical intensity modulator, the optical intensity modulator being responsive to an RF input signal and to the series of RF comb lines on the optical carrier generated by the photonic oscillator for generating a optical signal which is applied to said photodetector. 
   
   
     18. The agile spread spectrum waveform generator of  claim 1  further including an optical coupler responsive to an RF input signal, the optical coupler being connected to receive the series of RF comb lines on the optical carrier generated by the photonic oscillator and the frequency translation reference generated by the second laser, the optical coupler being connected either upstream or downstream of the optical intensity modulator which is responsive to the RF input signal. 
   
   
     19. The agile spread spectrum waveform generator of  claim 18  wherein the RF input signal includes a pulsed code or polyphased codes. 
   
   
     20. A method of generating an agile spread spectrum waveform, the method comprising:
 (a) generating multi-tone optical comb as a series of RF comb lines on an optical carrier using a photonic oscillator; 
 (b) generating a wavelength-tunable single tone frequency translation reference; and 
 (c) optically combining the optical comb with the frequency translation reference to generate a lightwave waveform suitable for subsequent heterodyning; 
 wherein generating multi-tone optical comb comprises:
 (i) optically delaying the comb in a first loop for spacing comb lines in the comb; 
 (ii) optically delaying the comb in a second loop line for noise reduction, a second optical delay caused by step (ii) being longer than a first optical delay caused by step (i); 
 (iii) photodetecting the delayed comb; and 
 (iv) using the delayed comb in an optical intensity modulator to modulate an output of a laser to thereby generate said multi-tone optical comb as a series of RF comb lines on an optical carrier. 
 
 
   
   
     21. The method of  claim 20  further including the step of heterodyning the lightwave waveform. 
   
   
     22. The method of  claim 21  further wherein the step of heterodyning is performed by at least one photodetector. 
   
   
     23. The method of  claim 20  wherein a loop common portion further includes an amplifier for amplifying the comb and a band pass filter for establishing a bandwidth of the comb. 
   
   
     24. The method of  claim 23  wherein the amplifying is performed electronically. 
   
   
     25. The method of  claim 20  wherein a loop common portion includes a band pass filter for establishing a band width of the comb and further including a step of optically amplifying the comb in at least one of the first and second loops. 
   
   
     26. The method of  claim 20  further including the step of compensating for environmental changes by changing the amount of at least one of the first and second optical delays. 
   
   
     27. The method of  claim 26  wherein the step of compensating for environmental changes by changing an amount of at least one of the optical delays is performed by comparing frequency or phase of one comb line in the comb with a reference and adjusting a length of at least one optical delay line carrying the comb. 
   
   
     28. The method of  claim 27  wherein the adjusting step adjusts the length of first and second optical delay lines. 
   
   
     29. The method of  claim 26  wherein the step of compensating for environmental changes by changing an amount of at least one of the optical delays is performed by comparing frequency or phase of one comb line in the comb with a reference and adjusting a phase of the comb. 
   
   
     30. The method of  claim 20  further including the step of seeding the photonic oscillator to initiate the comb. 
   
   
     31. The method of  claim 20  wherein the second optical delay is more than 40 times longer than is the first optical delay. 
   
   
     32. The method of  claim 20  further including the step of intensity modulating the comb with an optical intensity modulator responsive to an RF input signal and to the series of RF comb lines on the optical carrier for modulating said lightwave waveform. 
   
   
     33. The method of  claim 32  wherein the RF input signal applies a pulsed code or polyphased codes to the optical intensity modulator. 
   
   
     34. The method of  claim 20  further including the step of modulating the intensity of the comb and the frequency translation reference with an optical intensity modulator responsive to an RF input signal and to the series of RF comb lines on the optical carrier and to the frequency translation reference for modulating said lightwave waveform. 
   
   
     35. The method of  claim 34  wherein the RF input signal applies a pulsed code or polyphased codes to the optical intensity modulator.